Pin 2 of each connector (i.e. the ones facing the edge of the board, by the L and R labels) is positive, in the sense that the signal there will be in phase with the line output.
That said, in practice it probably doesn't matter too much as long as both speakers are connected in the same direction. And for future reference, each amp IC contains two amplifiers in a bridged configuration, so both output pins are active, with opposite phase, meaning neither pin can be connected to system ground.